Notebook template format and example notebooks

The `jupyter-notebook` repository comes with example notebooks, and the same is true for most nbextensions, like `ipywidgets`.
- create a new file extension for notebook templates, that cannot be modified in-place. (When opening them, we would get a copy of it).
- have a `install_example_notebooks` helper function (like `install_nbextension`) that puts the example notebooks (templates) of a given package into a dedicated directory, and make the templates always be accessible from the dashboard regardless of the current working directory.